The unusual large-scale morphological and kinematical features that
are found in the radio and optical observations of a few galaxies have
been interpreted by some authors as the consequence of the expulsion
of gas at high speeds from the nuclei. Two classic examples are M82
and NGC 1275
(Burbidge et al. 1963).
However, the increasing
complexity of the observational data obtained in the last few years
has led to an increasing diversity of explanations: The peculiar
appearance of M82 has been ascribed to an encounter of the galaxy with
a cloud of gas, possibly swept up during a recent close interaction
with M81
(Gottesman & Weliachew
1977;
see also
Cottrell 1977).
Solinger et al. (1977)
consider it possible that M82 has
recently captured a cloud of intergalactic dust. In the case of NGC 1275,
Rubin et al. (1977)
have suggested that the high-velocity gas
may be interpreted as constituting an intervening late-type galaxy.

Perhaps the best case for large-scale effects of nuclear explosions
is to be made with NGC 4258. The broad, extensive, anomalous radio
continuum and H arms
have been analyzed in some detail as arising
from the interaction of gas expelled from the nucleus with gas further
out in the disk
(van der Kruit et
al. 1972).
Later observations of the
kinematics of the gas provide at least a qualitative agreement with
the predictions of the model
(van der Kruit 1974b,
van Albada & Shane
1975),
although the rather modest disturbance of the HI distribution
and kinematics is somewhat disappointing considering the spectacular
nature of the radio continuum picture. Further discussion of this
galaxy can be found in the papers by
Oort (1974,
1975).

A number of galaxies selected on the basis of their radio continuum
morphology have been offered as examples in this class by
de Bruyn (1977b),
who also estimated the lifetimes of the observable features
as 107 to 5 × 108 years. The fact that about
10% of the luminous massive
spirals show evidence for this sort of activity could then be
interpreted to mean that every large disk galaxy experiences
recurrences of these phenomena every 108 to
5 × 109 years.